O,O,S-Trimethyl phosphorothioate, an impurity present in technical organophosphorus insecticides, has recently been shown to cause delayed death in rats at low dosages. Work is proposed to examine other trialkyl phosphorothioates and related esters for delayed toxicity in rats and other animals and to elucidate their mode of action. A variety of trialkyl phosphorothioates and phosphorodithioates will be synthesized and evaluated for delayed toxicity in mammals to determine the basic structural requirements necessary for toxicity. The effect of chirality of the phosphorus atom on toxicity will be determined. Studies to be conducted to determine the mode of action of these compounds include examination of both in vivo and in vitro inhibition of different esterases present in different rat tissues, including serum, erythrocyte and brain cholinesterase, and serum and liver carboxylesterase. The effect of the antagonist, O,O,O-trimethyl phosphorothioate, on esterase inhibition also will be examined. Either l4C or 32P O,O,S-trimethyl phosphorothioate will be synthesized to determine tissue localization of radioactivity and tissues with high concentrations of radioactivity will be examined for esterases. The effect of atropine, 2-PAM and other therapeutic agents on the toxicity of the O,O,S-trimethyl ester will be determined. Inhibition of chymotrypsin, trypsin and other proteolases and hydrolases will be examined.